The invention is related to filaments. In particular, the invention is related to filament construction for electronic emitters.
A filament comprises at least one emitter. An emitter is a component that releases energy, as in the form of electrons, upon the absorption of energy. In the filament, the emitter is one element and the filament can include additional features. Alternatively, the filament can comprise a plurality of emitters.
Conventional filament designs for lighting and electronic emission generally comprise a helical coil geometry. While a helical coil has proven adequate for many applications that require relatively isotropic illumination, a helical coil may be inefficient for electronic emission. This inefficiency is partly due to space-charge limitations on emission current, which result in low saturation, and hence a weak signal. Additionally, a large fraction of electron trajectories reaches an associated anode outside a desired target area, leading to an undesirable focal spot profile.
The prior art in filaments, emitters, filament manufacture and support assemblies focuses on tungsten helical coil emitters. Attachment of helical coil filaments to supports is accomplished by crimping the filament wire inside electrically conducting leads. The techniques used in this method of attachment often result in filament misalignment, leading to undesirable focal spot characteristics.
Ribbon-like filaments, and their emitters, have been known in the art for illumination and electronic emission purposes. These ribbon filaments generally comprise a single emitter. These known ribbon filaments comprise integrally formed leads, and are thus difficult to attach to supports with a desired alignment accuracy. The integral-lead configuration compromises the filament alignment in a cathode assembly because the ribbon filaments are prone to warp as the integral leads are twisted during attachment to the support structure.
Near-isothermal heating is exhibited in sufficiently long helical coil filaments due to the coils possessing an extended length of uniform cross-section. The uniform cross-section results in essentially negligible heat conduction along a potion of the filament. Known ribbon filaments do not maintain a uniform temperature across the emitter and hence do not approach their potential thermionic emission current or life. Further, known ribbon filaments do not possess an engineered temperature distribution across the filament, and thus do not achieve their potential focal spot quality. Further deficiencies of known ribbon filaments include inadequate mounted stability and ease of alignment with a support and mounting structure,
It is therefore desirable to improve performance of filaments and associated emitters by introducing filament designs that produce desired temperature distributions across emitters and prolonged emitter life, while attaining high emission currents and good focal spot quality. Also, it is desirable to provide filament geometries that offer substantial mounting advantages over conventional helical coils. The mounting advantages include, but are not limited to, enhanced focusability, geometric stability, consequent durability and ease of alignment within a filament mounting structure, and retention of focal spot quality.
One aspect of the invention provides a method for determining a geometry of a filament. The filament is composed of a thin metal foil, ribbon or sheet, and that has a geometry that exhibits a prescribed temperature distribution across it, thus enhancing electron emission and life. The method comprises generating a three-dimensional (hereinafter xe2x80x9c3-Dxe2x80x9d) mesh of a filament geometry; imposing boundary conditions on the 3-D mesh; solving a coupled thermal-electrical equation to determine a temperature distribution across a surface of the generated filament geometry subject to imposed boundary conditions; and determining that the filament geometry is acceptable when temperature distribution specifications are met. If the filament geometry does not conform to the temperature distribution specifications, the filament geometry determination method is iterated until the temperature distribution is acceptable.
A filament that is formed from a thin metal foil, ribbon or sheet is provided, as another embodiment of the invention. The filament comprises at least one emitter that releases energy, generally in the form of electrons or photons, at least one current-crowding structure that confines current flow, and at least one tab on each end of an emitter for attachment of the emitter. The emitter further comprises additional tabs. Thus, when current is passed through the filament, the current-crowding structure establishes current flow through the filament, resulting in a desired temperature distribution across the emitter.
Another aspect of the invention includes a method of making a curved filament. The method comprises providing a thin metal foil, ribbon or sheet starting filament, having at least one emitter and defining axes. The filament includes at least one current-crowding structure, so when current is passed through the filament the current-crowding structure establishes the desired temperature distribution across the filament. The method includes the steps of providing a first stationary die; disposing the filament on the first stationary die; providing a movable die; moving the moveable die toward the filament; and deforming the filament to produce a desired curvature in the filament.
Still another embodiment of the invention includes a support system for a filament support, where the filament comprises at least one emitter having tabs. The system includes a plurality of leads comprising tab connectors that allow attachment to the plurality of filament tabs; and further a support structure comprising at least a plurality of attachment posts, each post comprising a slot adapted to receive a lead. Thus, when each tab is attached to a lead and each lead is attached to a post, the filament is mechanically and electrically supported.
The filament, as set forth by the invention, is thin. For example, a filament possesses a thickness in the range between about 0.01 mm to about 1.0 mm. The filament comprises an appropriate emissive material such as, but not limited to a material selected from: substantially pure tungsten, tantalum, rhenium, and alloys thereof; a doped material, for example but not limited to potassium-doped tungsten for improved creep resistance; and at least one particulate containing material, such as carbides or oxide-containing materials for enhanced mechanical properties; and at least one of lanthanated, ceriated, hafniated, and thoriated tungsten for enhanced thermionic emission.
These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout, discloses embodiments of the invention.